The only difference between Under and Over grip pull ups is (obviously) the grip. In this exercise, the bar is gripped in an underhand fashion (and, in my case, the grip is closer together). More commonly known as a "chin up", this exercise focuses more on the biceps and lower lats (back).
This exercise is meant to be done in addition to the over grip pull ups.
I usually start with over grips for a few (about 3) sets and then do the same number of under grips. The same spotting method from the last exercise is very valuable in this exercise as well.